Hybrid materials and methods for producing the same

ABSTRACT

A hybrid material is provided which comprises a first single-walled nanotube having a lumen, and a fill molecule contained within the lumen of the single-walled nanotube. A method for producing the hybrid material is also provided wherein a single-walled nanotube is contacted with a fill molecule to cause the fill molecule to enter the lumen of the single-walled nanotube.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 09/625,946, filedon Jul. 26, 2000, now U.S. Pat. No. 6,544,463, issued on Apr. 8, 2003,which claims the benefit of provisional Application No. 60/145,586,filed on Jul. 26, 1999, each of which is incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to single-walled nanotubes and methods forproducing the same. In particular, the present invention relates tosingle-walled nanotubes having filled or partially-filled lumens and tomethods for producing the same.

BACKGROUND OF THE INVENTION

Since their discovery, single-walled carbon nanotubes (SWCNTs) havestimulated widespread scientific research due to their promisingelectronic and mechanical properties. Proven techniques for theproduction of SWCNTs include carbon arc-discharge evaporation (Iijima etal., “Single-shell carbon nanotubes of 1-nm diameter”, NATURE, Vol. 363,pp. 603-604, Jun. 17, 1993; Ajayan et al., “Growth morphologies duringcobalt-catalyzed single-shell carbon nanotube synthesis”, Chem Phys.Lett., 215(5), pp. 509-517, Dec. 10, 1993; Bethune et al.,“Cobalt-catalysed growth of carbon nanotubes with single-atomic-layerwalls”, NATURE, vol. 363, pp. 605-606, Jun. 17, 1993), pulsed laservaporization (PLV) of graphite in the presence of certain metalliccatalysts (Thess et al., “Crystalline Ropes of Metallic CarbonNanotubes”, SCIENCE, vol. 273, pp. 483-487, Jul. 26, 1996), and chemicalvapor deposition.

The known methods for producing SWCNTs, however, are unable to producehigh quality SWCNT material. Accordingly, the material is refluxed innitric acid to destroy contaminants that are more easily oxidized thanthe SWCNTs and then filtered to yield a sheet of tangled nanotubes afraction of a millimeter thick (i.e., bucky paper). The bucky paper isthen vacuum annealed at 1100° C. to drive off C₆₀ molecules and otherresidual organic impurities.

SWCNTs, produced using a pulsed laser vaporization (PLV) process similarto the one described above, have been investigated using high resolutiontransmission electron microscopy (HRTEM). Since a carbon nanotubesatisfies the weak phase object approximation, its image is a projectionof the specimen potential onto a plane that lies normal to the electronbeam. The image has maximum contrast where the beam encounters the mostcarbon atoms, which occurs where the electron beam is tangent to thegraphene walls of the carbon nanotube. Indeed, when SWCNTs which arepulled away from the bulk bucky paper were examined using HRTEM, animage consisting of two dark parallel lines separated by about 1.4 nmwas obtained. The fact that the space between the parallel lines wasclear indicates that no material is present within the SWCNTs.Characterizations of purified PLV-produced nanotubes by X-raydiffraction and Raman spectroscopy confirm these results.

Presently, a SWCNT has the largest elastic modulus (i.e., stiffness) ofany intrinsic material. Accordingly, SWCNTs have been used to formreinforced polymer-matrix composites which are useful as high strength,light weight materials. In general, the mechanical properties of thereinforced composites are improved by increasing the stiffness of thereinforcing fibers. As a result, there is a continued need for carbonfibers having even larger elastic modulii.

It has been shown that the stiffness of SWCNTs can be increased byfilling it with certain small molecules. Like any other material, when aSWCNT is placed in tension, it undergoes a Poisson contraction thatcauses a reduction in diameter. By filling-SWCNTs with a molecule havinga small compressibility, such as C₆₀, the Poisson contraction isresisted and the elastic modulus is increased. This resistance totransverse deformation also indicates that SWCNTs containing C₆₀ shouldbe less likely to debond from the surrounding matrix, minimizing thelikelihood of fiber pull-out, a common failure mode in composites. Useof such filled SWCNTs as reinforcing fibers in polymer-matrix compositesshould therefore provide a means for improving upon the mechanicalproperties of polymer-matrix composites.

It has been shown that exterior adsorbates can change metallic SWCNTsinto semiconducting SWCNTs, opening the possibility for all-carbonmetal-semiconductor junctions at the nanometer length scale. Dopantsintercalated in between nanotubes have also been shown to affect thetubes' electronic properties. However, the usefulness of exteriormolecules to modify the intrinsic properties of nanotubes is limited bythe fact that exterior molecules are accessible to chemical reaction andmay be unstable in solvents, vacuum, or certain atmospheres. Interiormolecules are hermetically sealed within the nanotube cores, with thenanotube itself forming a steric and kinetic barrier to reaction. It isexpected that certain interior molecules, like exterior molecules, willmodify the electronic properties of nanotubes. Therefore, it would behighly beneficial to provide a means for permanently modifying theelectronic properties of nanotubes by filling to produce novel devices,interconnects, and other technologies.

Mass transport inside a nanotube has been demonstrated by the motion ofencapsulated C₆₀ along the axis of the surrounding tube. By theencapsulation of molecules having a strong magnetic moment,one-dimensional, nanoscopic mass transport devices which can be drivenby an applied magnetic field are possible. Since even strong magneticfields are known to be bio-compatible, this has potential application inthe targeted delivery of drug molecules by the field induced eject ionof the molecules from the nanotube cavity into the target (i.e., anano-syringe). Mass transport may also be driven by other means (e.g.,by momentum transfer from a laser). In all cases, the nanotube can beutilized to direct the motion of molecules in a controlled manner andalong specific pathways.

Nanotubes also show promise for use as chambers to produce or catalyzethe production of molecular structures that could not be easily producedotherwise. For example, a nanotube can act to enhance a reaction rate byconfining reactants in close proximity, and it can determine thereaction product by providing a steric constraint on its structuralform. Despite its small diameter, a nanotube is sufficiently strong tobe self supporting over distances that are large as compared to manymolecules. The nanotube therefore provides a convenient means tostabilize individual molecules for high signal-to-noise ratiocharacterization, such as by electron beam techniques, without theinterference from a substrate film.

In light of the foregoing, SWCNTs and other types of single-wallednanotubes (SWNTs) show promise as reinforcing fibers for polymer-matrixcomposites, novel electronic devices, and nanoscopic mass transportdevices (including nanoscopic drug delivery systems). The use ofnanotubes in such applications and others is dependent upon the abilityto controllably modify the intrinsic (e.g., mechanical, electronic,and/or magnetic) properties of the SWNTs by manipulating theirmicrostructure. A particularly promising means for altering theintrinsic properties of SWNTs and using SWNTs in novel ways involves thefilling of nanotube cavities or lumens to produce hybrid molecularassemblies that can have novel functionality. Accordingly, there is aneed for nanotubes containing small molecules within their interiorlumens and methods for producing the same.

SUMMARY OF THE INVENTION

The present invention relates to single-walled nanotubes having filledor partially filled lumens and methods for producing the same. Thesingle-walled nanotubes are filled or partially filled with molecularspecies that are expected to alter the microstructure of thesingle-walled nanotubes. As such, the intrinsic (e.g., mechanical,electronic, and/or magnetic) properties of the filled or partiallyfilled nanotubes can be controllably modified making the filled orpartially filled nanotubes useful as reinforcing fibers forpolymer-matrix composites, novel electronic devices, and nanoscopic masstransport devices.

In one of its aspects, the present invention relates to a hybridmaterial comprising a single-walled nanotube and a fill moleculecontained within a lumen of the nanotube.

In another of its aspects, the present invention relates to a bulkmaterial comprising nanotubes. The nanotubes are characterized by thefact that at least about 5%, preferably at least about 50%, and morepreferably at least about 80% of the nanotubes contain one or more fillmolecules within their lumens. In one embodiment, the nanotubes arearranged as a sheet of nanotubes.

In yet another of its aspects, the present invention relates to a methodfor producing a hybrid material comprising a single-walled nanotubehaving a lumen and a fill molecule contained within the lumen of thesingle-walled nanotube. The method comprises the step of forming aprecursor material. The precursor material comprises nanotubes producedusing any of a variety of techniques, including carbon arc-dischargeevaporation, chemical vapor deposition, and pulsed laser vaporization.The nanotubes are optionally purified or similarly treated. Theprecursor material is then contacted with a fill molecule to bring theprecursor material in physical contact with, or close proximity to, thefill molecules. In one embodiment, the precursor material is contactedwith the fill molecule under conditions of temperature, pressure, andtime sufficient to allow one or more fill molecules to at leastpartially fill the lumens of the nanotubes. The precursor material isthen optionally heat treated to further induce the fill molecules toenter the lumens of the SWNTs. In one embodiment, the precursor materialis heat treated by annealing the precursor material at a temperaturebelow about 1000° C., preferably below about 800° C., and mostpreferably below about 600° C.

Additional features and embodiments of the present invention will becomeapparent to those skilled in the art in view of the ensuing disclosureand appended claims.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a hybrid material comprising asingle-walled nanotube (SWNT) having a lumen and a fill moleculecontained within the lumen of the SWNT.

The SWNT is formed using any of a variety of techniques known in theart. For example, the SWNT can be formed using pulsed laser vaporization(PLV) as described, for example, in Rinzler et al., Appl. Phys. A 67,pp. 29-37 (1998), the disclosure of which is incorporated herein, in itsentirety, by reference. Alternatively, the SWNT can be synthesized usingchemical vapor deposition. In still another embodiment, the SWNT isformed using carbon arc-discharge evaporation as described, for example,in Iijima et al., “Single-shell carbon nanotubes of 1-nm diameter”,NATURE, Vol. 363, pp. 603-604, (1993); Ajayan et al., “Growthmorphologies during cobalt-catalyzed single-shell carbon nanotubesynthesis”, Chem Phys. Lett., 215(5), pp. 509-517, (1993); and Bethuneet al., “Cobalt-catalysed growth of carbon nanotubes withsingle-atomic-layer walls”, NATURE, vol. 363, pp. 605-606, (1993), thedisclosures of which are all incorporated herein, in their entireties,by reference. Any of a variety of single-walled nanotubes arecontemplated for use with the present invention, including withoutlimitation single-walled carbon nanotubes and boron-nitride nanotubes.

The fill molecule that is contained within the SWNT can take a varietyof forms. In one embodiment, the fill molecule comprises one or more C₆₀molecules. Since C₆₀ molecules have a diameter of about 0.7 nm and agraphitic van Der Waals spacing of about 0.3 nm, the C₆₀ molecules fitwithin the lumen of a SWNT having a diameter of about 1.4 nm withoutsignificant distortion from its equilibrium configuration. When the SWNTcontains more than one C₆₀, the structure resembles that of a peapod,where the SWNT is the “pod” and the C₆₀ molecules are the individual“peas.”

In another embodiment, the fill molecule comprises one or more capsuleshaped fullerenes in the form of a cylinder of carbon, preferably having10n carbon atoms, wherein n≧0. Each capsule is optionally capped at oneend, both ends, or neither end by hemispherical carbon “caps”. Thecarbon cap is essentially one half of a C₆₀ molecule. Accordingly, whenthe capsule is capped on both ends, the fullerene contains 60+10n carbonatoms. Further, when the capsule is capped on both ends, the capsule canbe described as a metastable form of a fullerene. It should be apparentthat as n increases, the capsule approximates a single-walled carbonnanotube, such that the hybrid material approaches a co-axial tube (CAT)structure.

In still another embodiment, the fill molecule comprises one or moremetallofullerenes. Metallofullernes are fullerenes containing any of avariety of chemical species, including one or more transition metalelements. For example, each fill molecule comprises two lanthinum atomssurrounded by, or endohedrally contained within, a C₈₀ cage (La₂@C₈₀).

The hybrid materials can be formed using a method in accordance with thepresent invention wherein the hybrid material is formed from a precursormaterial comprising one or more SWNTs. The SWNT is formed using any of avariety of techniques known in the art. For example, the SWNT can beformed using pulsed laser vaporization (PLV), chemical vapor deposition,or carbon arc-discharge evaporation.

Once the SWNT has been formed, the SWNT is optionally purified orsimilarly treated. For example, an acid (e.g., nitric acid) treatmentmay be imposed to remove contaminants that are more easily oxidized thanthe SWNTs. The SWNTs are then optionally filtered to recover the solid.

The precursor material is then contacted with the fill molecules tobring the precursor material in physical contact with, or closeproximity to, the fill molecules. In one embodiment, the precursormaterial is contacted with the fill molecules under conditions oftemperature, pressure and time sufficient to allow one or more fillmolecules to at least partially fill the lumens of the SWNTs. In anotherparticular embodiment, the fill molecules are themselves by-products ofthe process used to form the SWNT. For example, when the SWNT is formedby PLV and the resulting material is acid treated, the fill moleculesmay appear as a residue on the outer surfaces of the SWNT.

Alternatively, solution and/or vapor based transport can be used tocontact the SWNTs with the fill molecules. In the former case, a solvent(e.g., dimethyl formamide or toluene) is chosen to permit the moleculesto wash over and come in contact with the nanotubes. In the latter case,the temperature and/or pressure is tuned to permit the molecules toenter the vapor phase. These steps may be performed under vacuum, in aninert or non-oxidizing environment, or in air. The mobile molecules thencollide with the nanotubes and enter through openings in their walls,possibly assisted by diffusion along the nanotubes' exterior andinterior surfaces.

The precursor material is then optionally heat treated to further inducethe fill molecules to at least partially fill the lumens of the SWNTs.In one embodiment, the precursor material is annealed at a lowtemperature to produce the hybrid material. A minimum temperature ofabout 300° C., preferably about 325° C., and more preferably about 350°C. is achieved at a vacuum of about 3×10⁻⁵ Pa to promote exterior fillmolecules (e.g., C₆₀) to enter the tubes on a reasonable time scale.Additionally, the precursor material is annealed at a temperature belowabout 1000° C., preferably below about 800° C., and more preferablybelow about 600° C. Higher annealing temperatures limit the residencetime of C₆₀ on a SWNT as well as heal the nanotubes' walls, therebyeliminating access to their interiors. Since formation is governed byboth time and temperature, nanotube material is preferably soaked in,and not ramped through, this temperature window in order to producefilled SWNTs in abundance. The precursor material is annealed for a timesufficient to effectuate the production of the fill molecule within thelumen of the SWNT. For example, anneal times from about 1 hour to about100 hours, and preferably from about 1 hour to about 24 hours, can beutilized.

The filled or partially filled SWNTs are optionally further treated toinduce reactions between fill molecules within the lumens of the SWNTs.For example, when the SWNTs are filled or partially filled withfullerenes (e.g., C₆₀), heat treating the SWNTs at a temperature aboveabout 1000° C., preferably above about 1100°, and more preferably aboveabout 1200° C. causes the individual fullernes to coalesce to formextended fullerenes (e.g., capsules).

EXAMPLES

Small sections of nanotube material, which were torn away from a sheetof tangled SWCNTs (i.e., bucky paper) were investigated usingtransmission electron microscopy (TEM). The sections of nanotubematerial fixed inside a 3 mm slot grid to be placed inside a highresolution transmission electron microscope (HRTEM). At the tear, SWCNTsthat are pulled away from the bulk are suitable for imaging. Thispreparation technique does not subject the specimen to any additionalchemical or thermal processing.

Isolated SWCNTs were imaged in JEOL 4000EX and JEOL 2010F transmissionelectron microscopes at an accelerating voltage of 100 kV. Magnificationwas determined using polyaromatic carbon shells present in the specimen,whose lattice fringes have a well defined spacing of 0.34 nm.

The HRTEM image of a graphene is a projection of the specimen potentialonto a plane oriented perpendicular to the electron beam. The image hasmaximum contrast where the beam encounters the most carbon atoms, whichoccurs where it is tangent to the molecule's graphene-like walls. Thus,the image of a SWCNT consists of two dark parallel lines whoseseparation is equal to the tube's diameter, and the image of a C₆₀molecule is a circle 0.7 nm in diameter.

Examples 1 and 2

The nanotubes for Example 1 were prepared at Rice University in themanner described in A. G. Rinzler et al., Large-scale purification ofsingle-wall carbon nanotubes: process, product, and characterization,Appl. Phys. A 67 (1998) 323. In particular, nanotubes were synthesizedby the laser ablation of a graphitic target impregnated with 0.6 at %each Ni/Co catalyst. This raw nanotube “mat” or sheet was refluxed inHNO₃ for 48 hours, rinsed and neutralized, suspended in surfactant, andfiltered to form a thin paper. The nanotubes for Example 2 weresynthesized at the University of Monpellier by carbon arc (CA) dischargeusing 4.2/1.0 at % Ni/Y catalyst and then similarly purified. Such wetchemical etching is known to open the ends of nanotubes as well asattack their sidewalls.

A number of materials were examined by HRTEM in the as-synthesizedcondition, after acid purification, and after subsequent ex-situ orin-situ anneals under a vacuum of 20-40 μPa at temperatures of 100-1200°C. Temperature was monitored continuously via thermocouples. Duringin-situ experiments, only a few minutes were required to ramp betweentemperatures due to the small mass of the heater.

Baking as-purified material in vacuo at 225° C. for about 63 hoursproduces a general cleaning of the nanotubes. After baking, thenanotubes were free of most impurities, although X-ray diffraction andTEM observation confirm the presence of C₆₀ crystallites. The walls ofthe nanotubes appear as dark parallel lines in the HRTEM images and areseen to have breaks and disclinations that are the result of acidattack. Several hundred tubes were observed at various locations andwere found to be empty.

The nanotubes were then reannealed for 2 hours at 450° C. HRTEM imagesof the material showed that the walls of the tubes were partiallyhealed. Many tubes, both isolated and comprising ropes, were seen tocontain C₆₀ chains which appear as regularly spaced strings of 0.7 nmdiameter circles (C₆₀ molecules) between two parallel lines separated by1.4 nm (corresponding to the diameter of the surrounding nanotube).

The HRTEM images indicate that peapods are formed during the annealingtreatment following acid purification. Without being limited by theory,it is believed that the peapods are formed when residual exterior C₆₀arrives at nanotube ends or sidewall defects via surface diffusionand/or transport in the vapor phase. This phenomenon is demonstrated bythe following experiment. A specimen was prepared from PLV material andsubsequently cleaned by baking for 24 hours at 225° C. The temperaturewas then cycled in the range 225-375° C., equilibrating at 25°increments for observation. A sequence of HRTEM images was recorded at350° C. with 15-30 seconds between images. The nanotubes, which wereoriginally clean, were shown to have C₆₀ molecules adsorbed to theirsurface. The fullerenes adsorb to locations on the nanotubes for onlytransient times before vanishing so quickly such that the motion isundetectable to the eye. The occupied locations appear to be random.During exposure to the electron beam of the electron microscope,individual fullerenes are damaged and become fixed in position.

Mobility has been observed in situ only at furnace temperaturesexceeding 325° C. At the microscope column vacuum of 40 μPa, thesublimation temperature of solid C₆₀ is reported to be approximately375° C. The agreement of these temperatures suggests that C₆₀ is mostlikely to arrive at a nanotube via the vapor phase. At the sublimationtemperature of solid C₆₀, it might be expected that C₆₀ molecules aremore strongly bound to a SWCNT than to each other due to the greaternumber of carbon-carbon Van der Waals interactions in the former case.Thus once a fullerene arrives at a nanotube, surface diffusion to anopen end or sidewall defect could occur. As the temperature isincreased, the residence time of C₆₀ on the surface of the SWCNT—andthus the probability of that molecule entering the SWCNT—decreases. Oncethe C₆₀ molecules enter, the stabilizing Van der Waals coordination withthe surrounding tube causes them to be retained inside. Self-assemblyinto chains subsequently occurs because the interior fullerenes arestill mobile, thereby increasing each molecule's coordination evenfurther.

A test of the mechanism of peapod formation was conducted using acidpurified CA material, which is expected to have little exterior C₆₀ dueto its comparatively high catalyst concentration. Two specimens wereprepared from the as-received paper. The first served as a control,while a drop of C₆₀ suspended in dimethyl formamide was added to thesecond, which was then air dried. Preliminary TEM observation of bothsamples confirmed that the tubes were damaged with open ends, thatpeapods were absent, and that C₆₀ crystallites were sparse in thecontrol sample and abundant in the experimental sample. Each wasannealed for 1 hour at 400° C. Large numbers of peapods were found inthe experimental sample. In the control sample, a very low concentrationof peapods was found, consistent with the small amount of C₆₀ seen inthe as-purified CA material. In general, no difference could be detectedbetween the peapods produced in the PLV synthesized SWCNTs and those inthe CA synthesized SWCNTs.

The positive correlation between the amount of exterior C₆₀pre-annealing and the amount of interior C₆₀ post-annealing evidencesthat C₆₀ molecules enter the tubes during heat treatment. This iscorroborated by the fact that large (about 3 nm) diameter tubes areobserved to contain irregularly arranged clusters of C₆₀ near theirends. Encapsulated C₆₀ chains are expected in samples having a highexterior C₆₀ concentration, and intra-sample microsegregation is relatedto the initial distribution of C₆₀ throughout the bulk as well as to thetime and temperature of heat treatment.

Example 3

The related co-axial tube (CAT) structures, consisting of nested 0.7 nmand 1.4 nm diameter tubes, were synthesized by high-temperatureprocessing of C₆₀ chains. Consider that no CATs have been observed innanotube samples annealed in situ at temperatures up to 900° C., whilethey were initially discovered in material that was annealed above 1100°C. We have investigated this by annealing a PLV sample ex situ for 2hours at 450° C. in order to produce C₆₀ chains and subsequently holdingthe sample for 24 hours at 1200° C. After furnace cooling, the samplewas examined in the microscope, where CATs were observed in lieu ofinterior C₆₀. Without being bound by theory, it is believed that at1200° C., neighboring C₆₀ molecules have a reasonable probability ofcolliding with sufficient kinetic energy to cause them to coalescewithin the time frame of the experiment. The surrounding SWCNT thus actsas a reaction container, ensuring a near-zero impact parameter andtemplating the fused product into a 0.7 nm diameter SWCNT.

Example 4

SWCNTs containing metallofullerenes were also prepared. Nanotubematerial was impregnated with La₂@C₈₀ by adding a drop of La₂@C₈₀solution in toluene and CS₂ to a microgram-sized piece of acid purified,carbon arc prepared SWCNT paper (from P. Bernier, University ofMontpellier, and P. Petit, CNRS). Acid purification is known to open theends of nanotubes and introduce defects into their walls, providingaccess to their interior. After the toluene/CS₂ solvent was allowed toevaporate, the specimen was examined by HRTEM. The resulting image showsthe deposition of objects having circular cross sections, which arepresumably small aggregates of La₂@C₈₀ molecules. Substantially largerregions having obvious crystallinity were also observed in similarlyprepared samples. The acid etched nanotubes were found to be empty.Energy dispersive spectroscopy (EDS) of the bulk confirmed the presenceof La, which was not present in the CA material prior to impregnation.

The specimen was vacuum annealed in situ under about 20 μPa for 7 hoursat 400° C. and then for an additional 10 hours at 600° C. Following thisheat treatment, a large percentage of the SWCNTs were filled withclose-packed chains of metallofullerenes. An image of such an assemblyis a direct projection of the specimen potential. It is dark whereverthe electron beam is most strongly scattered, which occurs at thoselocations having the greatest mass-thickness in the direction of thebeam. The assembly appears as a row of circular features between twoparallel lines, corresponding to where the beam is tangent to the wallsof the fullerene cages and the surrounding nanotube, respectively.

It is seen that each fullerene contains two point scattering centersappearing as dark spots within the circular, projected images of the C₈₀shells. These are not centered within the cages but instead arepositioned in close proximity to the fullerenes' interior walls and areseen to move in a manner that is described below. This motion, the largesize of the film grain relative to the image, and the point resolutionof the microscope (0.19 nm) determine the accuracy of the measurement onthe atomic positions. Nevertheless, the contrast signature is that ofheavy single atoms. The observations evidence that the moleculescontained by the SWCNT are metallofullerenes. Since it is known that theapplied heat treatment does not produce metallofullerenes, theseendohedral molecules are positively identified as the La₂@C₈₀ additive.

The projected images of the C₈₀ shells appear circular, suggesting aspheroidal symmetry. Of the three lowest energy La₂@C₆₀ isomerspresented in Table 1, this observation is consistent only with isomer C(D_(2h) having an I_(h) cage). The alternative isomers in Table 1 (A andB) have oblong cages with aspect ratios of approximately 1.3-1.4. Due tothe small size of the confining SWCNT, oblong La₂@C₈₀ would be expectedto align with its long axis parallel to the tube axis. Such differencesbetween major and minor diameters are well within the resolution of theexperiment but are not detected. Thus, the experimental data corroborateprior calculations that the D_(2h) La₂@C₈₀ structure (isomer C) isthermodynamically and kinetically favored.

TABLE 1 Isomers of La₂@C₈₀. La₂@C₈₀ symmetry C₈₀ cage (optimized La-LaShape of Isomer symmetry geometry) separation cage A D₂ D₂ 4.407 ÅOblong B D_(5d) (D₅) D₅ 3.881 Å Oblong C I_(h) D_(2h) 3.655 Å Spheroidal

This spheroidal isomer having a diameter of about 0.8 nm has evidentlyentered the SWCNTs. However, measurements indicate that many of thefilled SWCNTs have a diameter of about 1.4 nm. Since the energeticallyfavored graphitic Van der Waals gap between the walls of the containedand surrounding molecules is nominally 0.34 nm, this requires (1)compression of the Van der Waals gap to accommodate an undistortedmetallofullerene, (2) distortion of the metallofullerene to accommodatethe favored separation, or (3) reduction of the expected Van der Waalsgap due to electronic effects. In any case, the energy cost must be lessthan the energetic stability gained by encapsulation of the La₂@C₈₀.

The apparent separation of La atoms within the C₈₀ cages as measuredfrom a TEM micrograph depends upon the alignment of the La-La axis withrespect to the electron beam. A well separated pair oriented parallel tothe beam would appear as a single scattering entity of atomic number 154(twice the mass-thickness of a single La atom) due to the projectioneffect of the microscope. It is possible to place a lower bound on themaximum La-La separation by determining the largest separation seen inthe data. A separation is measured by examining the intensity profile ofthe image along the La-La axis. Each profile contains two peakscorresponding to the La atoms. The reported separation is the distancebetween the centers of these peaks. Measured La-La separations rangefrom 0-5 Å, with the latter value representing the lower bound. In thisanalysis, it must be ascertained that the scattering of the carbon atomsin the cage wall, when combined with the scattering of the La atom, doesnot produce an apparent shift in the position of the La atom. This wastested using multislice calculations of TEM images of D_(2h) La₂@C₈₀(isomer C) using the Cerius² suite of programs. An optimized structurewas assumed, having La-La separation of 3.655 Å, with the electron beamdirected perpendicular to the La-La axis. The La-La separation in thesimulated image, as measured from the centers of the intensity maxima,was approximately 4.7 Å, indicating a 1 Å shift in apparent separationdue to imaging artifacts. (Note that the simulation was performed on anisolated metallofullerene, and so the expected minor effect of thesurrounding SWCNT has not been considered.) Since the largest empiricalmeasurements exceed this artifact-corrected value, we conclude that theactual La-La separation in La₂@C₈₀@SWCNT is larger than the theoreticalseparation in La₂@C₈₀.

Additionally, although the endohedral La atoms are mobile, they remainstable for transient moments such that they can be imaged during evenabout 0.8 second exposures. A tumbling motion is directly observed, withthe atoms circuiting around the inside of the C₈₀ cage. A “ratcheting”behavior is observed where the atoms tumble in discrete jumps. Given thelarger than expected La-La separation, it is possible that the SWCNTinduces the La atoms to lie closer to the C₈₀ cage, thereby increasingthe potential barrier for accessing certain endohedral sites andfreezing out free rotation at room temperature. Such a large effect ofthe SWCNT on the behavior of the encapsulated molecule must by mirroredby an effect of the molecule on the SWCNT. In this way, it is possiblethat the functionality of the SWCNT has been modified by theencapsulate.

Those skilled in the art will appreciate that numerous changes andmodifications may be made to the preferred embodiments of the inventionand that such changes and modifications may be made without departingfrom the spirit of the invention. For example, the present inventioncontemplates the use of any of a variety of types of nanotubes,including boron-nitride nanotubes. It is therefore intended that theappended claims cover all equivalent variations as fall within the truescope and spirit of the invention.

1. A method for producing a hybrid material comprising providing a precursor material comprising at least one single-walled nanotube having a lumen in contact with at least one fill molecule by means of a solution or vapor transport and annealing at least one single-walled nanotube in the presence of the fill molecule to provide said hybrid material.
 2. The method of claim 1 wherein the annealing step comprises annealing the single-walled nanotube at a temperature below about 1000° C.
 3. The method of claim 2 wherein the annealing step comprises annealing the single-walled nanotube at a temperature below about 800° C.
 4. The method of claim 3 wherein the annealing step comprises annealing the single-walled nanotube at a temperature below about 600° C.
 5. The method of claim 1 wherein the annealing step comprises annealing the single-walled nanotube at a temperature above about 300° C.
 6. The method of claim 1 wherein the annealing step comprises annealing the single-walled nanotube in a vacuum.
 7. The method of claim 1 wherein the annealing step comprises annealing the single-walled nanotube in an inert environment.
 8. The method of claim 1 wherein the annealing step comprises annealing the single-walled nanotube in a non-oxidizing environment.
 9. The method of claim 1 wherein the annealing step comprises annealing the single-walled nanotube in air.
 10. The method of claim 1 wherein the annealing step comprises annealing the single-walled nanotube for a time from about 1 hour to about 24 hours.
 11. The method of claim 10 comprising the step of treating the single-walled nanotube with an acid prior to the annealing step.
 12. The method of claim 1 wherein fill molecules penetrate the walls of said nanotubes. 